The man from the Sыreté nodded.

"Be well assured, that will be attended to. I shall return to Paris tonight. There may be something to be got out of Élise, Giselle's maid, now that we know a little more about the case. Also, I will check up Giselle's movements very carefully. It will be well to know where she has been during the summer. She was, I know, at Le Pinet once or twice. We may get information as to her contacts with some of the English people involved. Ah, yes, there is much to do."

They both looked at Poirot, who was absorbed in thought.

"You going to take a hand at all, M. Poirot?" asked Japp.

Poirot roused himself.

"Yes, I think I should like to accompany M. Fournier to Paris."

"Enchanté," said the Frenchman.

"What are you up to, I wonder?" asked Japp. He looked at Poirot curiously. "You've been very quiet over all this. Got some of your little ideas, eh?"

"One or two - one or two - but it is very difficult."

"Let's hear about it."

"One thing that worries me," said Poirot slowly, "is the place where the blowpipe was found."

"Naturally! It nearly got you locked up."

Poirot shook his head.

"I do not mean that. It is not because it was found pushed down beside my seat that it worries me - it was its being pushed down behind any seat."

"I don't see anything in that," said Japp. "Whoever did it had got to hide the thing somewhere. He couldn't risk its being found on him."

"Évidemment. But you may have noticed, my friend, when you examined the plane, that although the windows cannot be opened, there is in each of them a ventilator - a circle of small, round holes in the glass which can be opened or closed by turning a fan of glass. These holes are of a sufficient circumference to admit the passage of our blowpipe. What could be simpler than to get rid of the blowpipe that way? It falls to the earth beneath and it is extremely unlikely that it will ever be found."

"I can think of an objection to that - the murderer was afraid of being seen. If he pushed the blowpipe through the ventilator, someone might have noticed." "I see," said Poirot. "He was not afraid of being seen placing the blowpipe to his lips and dispatching the fatal dart, but he was afraid of being seen trying to push the blowpipe through the window!"

"Sounds absurd, I admit," said Japp, "but there it is. He did hide the blowpipe behind the cushion of a seat. We can't get away from that."

Poirot did not answer, and Fournier asked curiously:

"It gives you an idea, that?"

Poirot bowed his head assentingly.

"It gives rise to, say, a speculation in my mind."

With absent-minded fingers he straightened the unused ink-stand that Japp's impatient hand had set a little askew.

Then lifting his head sharply, he asked: